There are NMR diagnostic apparatus in which the density distribution of specified atomic nuclei, such as the atomic nuclei of hydrogen in certain tissues of a living body, can be measured in a non-invasive manner from outside of the object utilizing NMR phenomena to obtain useful information for medical diagnostic purposes.
There has been diagnostic nuclear magnetic resonance computerized tomography apparatus (NMR-CT) which apply such NMR techniques using computerized tomography (CT) apparatus. The density distribution images of specified atomic nuclei in an object's slice can be reconstructed by a computer on the basis of the projection data of the specified atomic nuclei density distribution in the various directions on the object's slice that are acquired with NMR techniques.
One example of conventional NMR diagnostic apparatus is described in U.S. Pat. No. 4,254,778.
First, a static magnetic field is generated by four electromagnetic coils C1 shown in FIGS. 1(a) and 1(b). A gradient magnetic field is generated by gradient magnetic field coils C2. C3 and C4 shown in FIGS. 2 and 3 are superimposed on the static field. Coils C2, C3 and C4 are installed to be combined with coils C1.
Such a situation is schematically illustrated in FIG. 4. Initially, the static field Hzo due to coils C1 is applied to the object with the gradient field Gz due to coils C2. The gradient field Gz is established by flowing current in the reverse direction in a pair of coils of the Helmholtz type shown in FIG. 2. Therefore, the direction of its magnetic force lines is the same (the Z-direction) as that of the static field Hzo. Also, the strength of the gradient field is zero at the center plane between the two coils C2, and its absolute value increases linearly from the center plane to either side thereof, but the direction of the field strength is opposite one another.
Next, when a selective exciting pulse H1 having an appropriate frequency component is applied to the composite magnetic field through a pair of probe-head coils C5 shown in FIG. 5, resonance occurs only in a plane in which a resonance frequency determined by the local field due to the fields Hzo and Gz is equivalent to the frequency of the pulse H1. A gradient magnetic field GR formed by a combination of the magnetic fields GX and GY is applied to the plane of resonance. Measured through coils C5 is a free induction decay (FID) signal which is Fourier transformed to obtain the projection data of density distribution of atomic nuclei, for example, hydrogen along the gradient field GR in the selected plane. If projection data along a variety of directions is obtained by shifting the gradient field GR direction, the hydrogen nuclei density distribution image in a slice of the object is reconstructed using well-known techniques of X-ray computerized tomography.
On the other hand, the multi-sensitive point imaging technique is known as one technique which does not require the above-mentioned image reconstruction as is discussed in Hinshaw, Journal of Applied Physics, Vol. 47, No. 8, August 1976. Such a technique operates on the principle that the gradient magnetic field is oscillated by flowing an alternate current into the gradient magnetic field coils and in the meantime FID signals are continuously measured for integration, resulting in signals only on the center line in which the gradient field is not changed with time.
However it is extremely difficult to obtain a distinct selectively excited slice or tomographic plane (which is in fact constituted as a planar region of certain thickness) by the above-mentioned methods.
That is, in the case of the selective exciting method, forming completely selective exciting pulses is in fact substantially impossible while in the case of the multi-sensitive point method the magnitude of the magnetic field is successively varied with the position. Therefore, according to both methods the border or interface between the region where the magnetic field is not varied and the neighboring region is indefinite, thereby resulting in a blur of the selected plane.